Seasonal variations in the sporulating capacity of the fungus causing coffee berry disease

Assessments of the sporulating capacity of Colletotrichum coffeanum Noack in coffee branches (defined as the number of conidia produced per cm²per hour under optimum conditions) have been made for 4 years on a number of different sites in Kenya. An annual rhythm is evident which is essentially similar on all sites and for all years. Early-season and late-season peaks are separated by a mid-season trough. The change in sporulating capacity at any time is negatively correlated with wetness in the immediately preceding period, and daily rainfall can be used as a measure of wetness. The position of the peaks and their duration is defined by the timing of the onset and the duration of the monsoon rains, and is, therefore, subject to considerable variation. It is suggested that the correct timing of control sprays might be related to the form of the annual curve of sporulating capacity, and be predictable from rainfall records.     

The Relationship between Branch Canker and Twig Dieback of Tea Caused by Macrophoma theicola

During a recent survey of the inoculum source of twig dieback of tea caused by Macrophoma theicola, symptoms of canker were observed on branches of a number of tea cultivars. Most cankered areas contained minute black pycnidia of M. theicola, which ooze white conidial masses in strands when incubated under moist conditions. Macrophoma theicola was also consistently isolated from cankered branches. When healthy branches of tea were inoculated with conidia of M. theicola obtained from diseased branches, cankers similar to those occurring in nature developed in the inoculated areas, and M. theicola was re‐isolated from all experimentally induced cankers. Isolates derived from conidia of M. theicola also caused dieback on healthy tea twigs, indicating that fruiting bodies on the cankered branches are the main inoculum source of twig dieback of tea. There was a positive correlation between the severity of twig dieback and the prevalence of branch canker among the tea cultivars surveyed. Six of the cultivars surveyed were highly resistant to M. theicola.     

The development of silver scurf (Helminthosporium solani) disease of potato

The main source of inoculum of Helminthosporium solani was the seed tuber; the fungus was not detected in soils in which infected potatoes had been grown the previous year. Lesions spread rapidly on seed tubers after planting, so that within a few weeks the entire surface was covered. As lesions aged they lost the capacity to sporulate, so most inoculum was produced in the first few weeks after planting. Infection of progeny tubers was first detected at the heel (stolon attachment) end soon after their initiation. Once established, lesions spread slowly on the surface of progeny tubers when in the soil; more rapid spread took place during storage. Potatoes which seemed unaffected at lifting became diseased during storage.     

Water-borne spore dispersal in coffee berry disease and its relation to control*

Devices are described which were used to catch rain-dispersed Colletotrichum conidia within and between coffee trees infected with coffee berry disease (CBD). The amount of CBD inoculum dispersed was related to the number of diseased and sporulating berries but not to the sporulating capacity of the fungus in maturing bark. Wet conditions encouraged spore production and dispersal, and spraying with fungicides prolonged sporulation of diseased berries. The amount of pathogenic inoculum, subsequent disease development and crop losses were greatly increased by the presence of only a few diseased berries during growth of the young crop. Spore concentrations were greatest close to diseased berries, but most spores within trees were dispersed downwards through the canopy. Some spores were dispersed between trees by wind-driven rain-splash and vectors such as pickers. Inoculum sources in tree tops are most important during CBD epidemics and disease can be restricted by removing or spraying the tops of multiple-stem trees. Fungicide applied to tree tops effectively controls CBD, because spores dispersed during rain are accompanied by redistributed fungicide     

The stimulating effect of some fungicides on Glomerella cingulata in relation to the control of coffee berry disease

The non-parasitic strain of Colletotrichum coffeanum Noack which colonizes the maturing wood of Coffea arabica produces its perfect stage (Glomerella cingulata) on the branches. Assessments of the relative abundance of this in relation to preceding rainfall have shown that for unsprayed branches, but rarely for sprayed ones, the correlation is negative and highly significant. Various fungicides stimulate G. cingulata greatly, and this stimulation persists for many months. One other saprophytic component of the micro–flora is similarly stimulated, but with two others all the fungicides used depress sporulation. It is suggested that when the main source of infection of coffee berries is the pathogenic conidia from the wood, the mechanism of long–term control by early–season sprays may lie partly in this stimulation of some fungal competitors of the pathogen.     

Inoculum sources for coffee berry disease*

Isolates of Colletotrichum spp. from coffee berries and bark in Kenya fell into four categories, distinguishable by their cultural characteristics on agar. Three of these proved non-pathogenic; the fourth invariably infected both wounded and unwounded berries and caused coffee berry disease. The sporulating capacity on bark of the various Colletotrichum strains showed peaks in the periods before both the long and the short rains. The pathogenic strain made up only a very small proportion of the whole Colletotrichum complex on the bark and its sporulating capacity never exceeded twenty spores/cm²/h. By contrast, a green berry with an active coffee berry disease lesion produced 5×10⁴conidia/h and an infected ripe berry twice as many. It is concluded that in any year, regardless of cropping pattern, diseased berries are likely to play an important role in the development of coffee berry disease.     

Control of coffee berry disease*

Field experiments comparing six spraying programmes and two fungicides for control of coffee berry disease were carried out at three sites during 1968. These established that protection of the crop by sprays applied throughout the long rains (February–July) gave the best control and increased yields threefold. Early season spray programmes, which reduced the sporulating capacity of the pathogen in coffee twigs, but which ceased before the end of the rainy season, gave no disease control in the late crop (harvested October-December) and yields were less than in unsprayed controls. All programmes which gave protection at the onset of the rains gave some disease control in the early crop (harvested June-August). Captafol (‘Ortho-Difolatan’ 80 WP) was better than a 50% copper formulation (‘Perenox’). Conditions suitable for infection, as determined from meteorological records, occurred frequently during both rainy seasons. Estimates of sporulating capacity and data from spore trappings showed that although inoculum from the bark might initiate an epidemic, subsequent disease progress was more dependent upon spores derived from diseased berries; these accounted for most of the inoculum dispersed during the greater part of the season. Removal of the early crop, which was very small, had no detectable effect on disease in the late crop. Although losses were greatest with early season spray programmes, these did nevertheless delay the onset of the epidemic. However, the subsequent rate of disease increase was greatest and ‘scab’ lesion formation least in these treatments. ‘Scab’ lesions were most abundant in those treatments which caused least delay; in unsprayed coffee, it appeared that early exposure to infection resulted in ‘scab’ lesion formation which subsequently retarded the rate of disease progress. Tree height was positively correlated with disease incidence, but there was little effect of tree aspect. Crop density had a small effect on disease incidence within branches.     

Refining the biological factors affecting virulence of Botryosphaeriaceae on grapevines

Botryosphaeriaceae isolates of six species were assessed for their potential to infect grapevine tissues other than their tissues of isolation, primarily to determine sources of inocula that could contribute to bunch rot. Pathogenicity tests were conducted in vitro on berries and wood and in vivo on dormant buds of cultivars Chardonnay and Shiraz in glasshouse and field experiments. Tissue specificity and variation in virulence for different isolates was assessed. All isolates were able to infect and cause symptoms on detached 1‐year‐old canes and mature berries. Virulence was not affected by origin tissue and varied between isolates and within species. Inoculation of dormant buds did not affect bud burst or further development of shoots and fruit, however, a small number of Botryosphaeriaceae were reisolated from bunches at later growth stages. We conclude that Botryosphaeriaceae species are important pathogens of both the vegetative tissues and wood of grapevines. Grapevine wood infected with Botryosphaeriaceae could act as a source of inoculum for reproductive and vegetative tissue. Equally, the vegetative and reproductive tissues can also act as inoculum sources for wood infection. Therefore, all sources of inocula should be taken into consideration when developing management strategies for Botryosphaeria bunch rot and Botryosphaeria canker diseases.     

Effect of Ground Cover on Splash Dispersal of Phytophthora cactorum from Strawberry Fruits

Abstract A rain simulator was used to investigate the influence of various ground covers (soil, straw, sand, and plastic) on splash dispersal of Phytophthora cactorum , the cause of leather rot of strawberry. Potted strawberry plants were held in a concentric circle by a wood frame, 30 cm from an inoculum source of 10 infected fruits with sporulating lesions. The canopy was exposed to 15 and 30 min of generated rain at intensities of 15 and 30 mm/h. Plastic mulch had the highest resulting fruit disease, incidence (e.g., ca. 80% of fruit infected at 30 min duration and 15 mm/h rain), followed by soil and sand (36–47%), then straw (ca. 15%). Except for straw, there was lower incidence at 30 than at 15 mm/h. There was a general increase in disease incidence over time.     

Some epidemiological aspects of post-bloom fruit drop disease (Colletotrichum gloeosporioides) in citrus

Fluctuations in the incidence and amount of post-bloom fruit drop disease of citrus caused by Colletotrichum gloeosporioides in Belize prevent economic disease control. During the cooler drier months of the year when blossom infection is common there are variations in the incidence of rainfall and associated climatic parameters, and in the pattern of flowering. Large amounts of disease develop when periods of rain followed by prolonged wetness occur during peak blossoming periods. Blossoms are most susceptible during the open flower stage and infection of terminal flowers invariably results in infection of all other flowers on the spike. Disease incidence is greater in the lower parts of the trees, but flowering is greater in the upper regions. Large numbers of Colletotrichum spores are produced during wet conditions from apparently healthy leaves and from diseased flowers, but these rapidly lose viability when dried. Few spores are produced from old persistent calices (buttons). Although spores from leaves were a less potent inoculum source than those from flowers, they could provide the initial inoculum to commence flower infection when blossoming starts.     

Epidemiology of grey mildew and Alternaria blight of cotton

Grey (Areolate) mildew (Ramularia areola) and Alternaria blight (Alternaria macrospora) are two important fungal foliar diseases affecting cotton in India. Both the diseases are polycyclic in nature. The primary inoculum for grey mildew is through conidia or ascospores from infected debris and/or perennial cottons and the secondary spread is through primarily infected leaves. Whereas for Alternaria blight the spread is initially from seed-borne inoculum (in Gossypium herbaceum and Gossypium arboreum cottons) and/or crop debris and the secondary spread is from sporulating lesions on older leaves. Both R. areola and A. macrospora require a temperature regime of 20–30?°C with prolonged high humidity (>80%) and frequent rains for infection and disease development. However, it has been observed that cool weather coupled with prolonged dewy periods in the absence of rains has also been found conducive for the development of both the diseases. So, suitable epidemiological tools and models are required to predict the disease development, spread and to design suitable management practices.     

Relationship between inoculum density and vector phenology on the incidence of potato virus Y in tobacco

The epidemiology of potato virus Y (PVY) in the tobacco crop, Nicotiana tabacum, was examined in the context of the seasonal abundance of aphid vectors, rate of disease progress, and disease gradient from a known virus source. The spring potato crop, Solanum tuberosum, was suspected of being the main source of inoculum; therefore, varying numbers of infected potato plants were used as the inoculum source in different test plots. A 3-wk lag phase was present in all disease progress curves prior to an exponential increase in disease incidence. The relatively low numbers of aphid vectors, primarily transient species, alighting on the crop during the lag phase were responsible for the primary spread of PVY from potato to tobacco. The arrival of large numbers of colonising aphid vectors, Myzus persicae, presumably from the harvested potatoes, coincided with the exponential increase in PVY incidence in tobacco. The initial number of potato plants infected with PVY was positively correlated with the final disease incidence, rate of disease progress, and the magnitude of radial dispersion of PVY into the tobacco. Aphid vector pressure was not a significant variable in the differences in spatial and temporal characteristics of PVY epidemics among test plots.     

Analysis of Variance-Mean Relationships of Plant Diseases

Disease variance-mean relationships and the underlying mechanisms were examined using three approaches. First, data on four foliar diseases were collected, the variance of a disease increased geometrically, reached the maximum, and decreased geometrically when disease means were low, mediate, and high, respectively. There were great variations of variance values in the mediate ranges. Second, it was mathematically proven that the variance of a disease has an upper limit of variance (V_max)that follows a quadratic function. When a disease is rated in a range from 0 to C, the upper limit of variance (V_max) follows a quadratic function as V_max=CM-M² where M is disease mean. For the scale 0–10 or percentage [0–100]. the functions are V_max= 10M-M² or V_max=100M-M², respectively. A variance-mean relationship is dis-tributed within the region defined by the V,_max function. Third, a spatio-temporal simulation model was used to examine the effect of three spatial components, dispersal capacity, aggregation of initial inoculum, and geographic distribution of initial inoculum, on disease variance-mean relationship. The variance values inreased as the dispersal capacity decreased. The variance values inreased when pattern of mitial inoculum changed from regular pattern to aggregation or as the degree of aggregation increased. There was an interactive, effect between the two components on the variance values. Furthermore, when epi-demics started from situations having the same degree of aggregation but different geographic distributions for initial inoculum, the variance-mean relationships weredifferent. Variance values varied greatly even the degree of aggregation was constant. The inportance of these results in relation to interpretation of field experiments was discussed.     

THE EMERGENCE AND DEVELOPMENT OF BRANCHES IN CROTALARIA JUNCEA AND THEIR RELATIONSHIP TO FLOWERING

Nanda , K. K. (Forest Res. Inst., Dehra Dun, India.) The emergence and development of branches in Crotalaria juncea and their relationship to flowering. Amer. Jour. Bot. 49(4): 334–341. Illus. 1962.—Seeds of Crotalaria juncea L. were sown in pots on March 23, 1959, and records were kept of the dates of emergence of individual branches and the appearance of flower buds on them. Periodical observations were also made of the height of the main shoot as well as its branches and the number of nodes and leaves borne by them throughout the year. The main shoot elongates rapidly and terminates in an inflorescence. Development of lateral buds remains completely arrested during the period of rapid elongation and is initiated only after the appearance of the floral buds when it takes place in basipetal sequence. The flowering of the branches also takes place in a basipetal manner. This mode of emergence of branches and their flowering are exhibited even by secondary and tertiary branches. The length attained by these branches is very small as the flower buds appear soon after their emergence. In contrast, the branches formed towards the middle of May continue to elongate for a considerable period and become many times longer than the main shoot or the branches produced earlier in the season. The vegetative period of these branches is also very much prolonged. These differences in the height attained by branches produced at different times of the year and the basipetal sequence in the emergence of branches and their flowering appear to be under the control of some physio-chemical changes which cause the transformation of the growing apex from the vegetative to the reproductive state. This holds good irrespective of whether these changes are brought about as a result of a favorable environmental complex, as is the case with the main shoot and late-formed, much elongated branches, or are due to the favorable internal conditions produced within the plant as a result of the completion of the developmental process of the main shoot, as happens in the case of branches produced earlier in the season.     

Effects of Long‐Term Storage Methods on the Infectivity of Urediospores of Phakopsora pachyrhizi

A method for long‐term storage of spores of Phakopsora pachyrhizi was optimized. Three methods with different procedures for spore harvest and four different reactivation methods (varying in hydration or using heat shock) were analysed for the suitability for long‐term storage at ?80°C. All conservation methods as well as all reactivation methods lead to the infection of soybean leaves after 1 year of storage. Regarding efficiency and labour input, the most recommended method is to tap off spores from infected and sporulating leaves with subsequent dehydration before storage at ?80°C. Because hydration or heat shock steps did not provide any advantages, spores can be suspended in Tween water directly after storage and used as inoculum.     

Seasonal changes in primary and secondary inoculum during epidemics of leaf blotch (Rhynchosporium secalis) on winter barley

Seasonal changes in numbers of conidia of Rhynchosporium secalis on debris from previous barley crops infected with leaf blotch (primary inoculum) were monitored in 1985–86 and 1986–87. In 1986–87, changes in numbers of conidia on leaves of plants in the new winter barley crop (secondary inoculum) were also recorded. The greatest increases in production of primary inoculum were in early spring after rain, when temperatures were increasing after periods of sub-zero temperatures when there was little conidial production. Subsequently, more conidia were recovered from this debris after cycles of drying and rewetting than when it remained wet. After January 1987, amounts of secondary inoculum produced on the crop were much greater than amounts of primary inoculum on debris. Most spores were produced on the basal leaves and more spores were present on the September-sown than on the November-sown crop. Thus, while primary inoculum was a source of disease when plants were emerging, secondary inoculum on basal leaves was the main source of disease at stem extension, especially on early-sown crops.     

Formation and Development of Pseudothecia of Venturia nashicola

Conidia are believed to be the main source of primary inoculum for pear scab, caused by Venturia nashicola, in northern China. Experiments were conducted to investigate the development and potential role of V. nashicola ascospores in northern China. Leaves with pear scab lesions were collected from commercial orchards in November 2003 and 2004 to monitor pseudothecia formation under various environments. Pseudothecium production was shown to occur readily in northern China. The key requirement for pseudothecium production is the occurrence of rain during the winter and early spring, although the exact timing of these rain events appeared not to affect their development. Excess water may lead to the accelerated leaf decay and hence lead to production of fewer pseudothecia. More than 80% scabbed leaves, placed in a pear orchard, produced pseudothecia. Leaves with only non‐sporulating scab lesions in autumn were also able to produce a large number of pseudothecia. Both airborne ascospores and conidia of V. nashicola were caught in a pear orchard. Most ascospores were released by late‐May, a month after pear blossom. These results suggest that ascospores may play an important role in the early stage of pear scab epidemics in spring in northern China.     

Adapting liquid spawn Lentinus edodes to oak wood

Summary Lentinus edodes (Berk.) Sing. is a commercially important edible mushroom cultivated on oak (Quercus spp.) logs or wood particles. Inoculum (spawn) of this white-rot saprophyte, made by homogenizing cultures grown on a chemically defined liquid medium, dies when transferred to oak wood. This study shows that an aqueous extract of the oak wood is inhibitory to growth. Inoculum produced by homogenizing stationary-grown cultures was particularly sensitive to the extract. Three different methods decreased the sensitivity of inoculum: (1) use of intact (nonhomogenized) cultures as inoculum, (2) use of homogenized cultures produced from shaken rather than stationary cultures as inoculum, or (3) use of cultures grown in the presence of wood extract as inoculum. With the use of shaken cultures adapted to oak wood extract, homogenized-culture inoculum successfully colonized an oak wood medium. Adaptation of the fungus to extracts made from an intended substrate may be an inexpensive way of producing liquid spawn specifically suited for the inoculation of the substrate.This article was written and prepared by U.S. Government employees on official time, and it is therefore in the public domain (i.e., it cannot be copyrighted).Research carried out while T. J. Griffin was a volunteer worker at Forest Products Laboratory     

Effect of certain herbicide treatments on pasture composition and inoculum of the take-all fungus

The improvement of pastures by the use of a range of herbicides to eliminate grasses, and their effect on populations of the take-all fungus (Gaeumannomyces graminis vartritici=Ggt) were studied in the field (at Esperance Downs, on the south-coast of Western Australia) from 1982 to 1985. Field trials were conducted to evaluate three herbicide treatments (2,4-D amine+propyzamide; 2,4-D amine+paraquat; paraquat/ diquat) and an unsprayed control. A pot trial involving these treatments with two levels of nitrogen was undertaken to confirm treatment effects observed in the field trial. All herbicide treatments resulted in reduced grass composition of pastures, in both the year of spraying and in the second year of pasture, but reduced dry matter production in the year of spraying. In the year of spraying, however, inoculum ofGgt was reduced (P<0.1) only following the 2,4-D amine+propyzamide treatment and was greater (P<0.1) after 2,4-D amine+paraquat treatment than the unsprayed treatment. Despite reduced grass levels in the herbicide-treated plots in the second year of pasture,Ggt inoculum did not differ between treatments, nor did it after a wheat crop which followed a second year pasture. There was high correlation (P<0.001) between disease levels and dry weights of grasses in the pot trial. There was significantly less (P<0.001) grass in pots treated with herbicides compared to the unsprayed control but no difference (P>0.05) was evident between treatments. Inoculum levels were lower (P<0.05) in the treated pots than the unsprayed control with no evidence of differences among treatments (P>0.05). Nitrogen level had no effect on disease (P>0.05). All herbicide treatments tested reduced grass level and total dry matter, both in the field and in pots. Whereas in the pot trial reduced grass levels resulted in reducedGgt inoculum, in the field such a reduction occurred only with the 2,4-D amine+propyzamide treatment and only in the year of spraying. Herbicide treatments had no effect onGgt inoculum in second year of pasture or crop. Unknown soil and environmental factors in the field precluded a simple relationship between grass level in pasture and subsequent level ofGgt inoculum, and where such a relationship did occur (2,4-D amine+propyzamide treatment) it appeared to be shortlived.     

Effect of Inoculum Density on Verticillium Wilt Incidence in Commercial Olive Orchards

Verticillium wilt, caused by Verticillium dahliae Kleb., is presently the most destructive disease of olive, particularly in Andalucía (southern Spain). ‘Picual’ and ‘Arbequina’ are the dominant cultivars being planted in Spain. Both cultivars are highly susceptible to the defoliating pathotype of V. dahliae when artificially inoculated by root‐dipping or stem injection. Conversely, ‘Arbequina’ is considered more resistant than ‘Picual’ based on field observations and farmer's experience. In this study, the differential reaction between of cultivars was confirmed by surveys of naturally infested orchards with different inoculum densities of the pathogen. The average percentage of affected olive trees of ‘Picual’ was 60.2%, while only 13.1% of trees of ‘Arbequina’ showed disease symptoms. Overall, the pathogen caused extensive wilting of branches and defoliation on the trees of ‘Picual’, whereas ‘Arbequina’‐infected trees showed chlorotic symptoms and slight defoliation. The relationship between inoculum density and disease incidence fit a logarithmic function for both cultivars. The percentage of affected trees of ‘Arbequina’ per year increased linearly (y = 0.3559x, R² = 0.5652, and P = 0.0195) with the inoculum density in the soil, whereas this relationship was not observed for the ‘Picual’. Planting density had no effect on disease incidence for any of the two cultivars.